Fix asyncio.get_event_loop() documentation (GH-18051)

Mention that the function implicitly creates new event loop only if called from the main thread.
(cherry picked from commit 2c49becc69)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Doc/library/asyncio-eventloop.rst

@@ -34,8 +34,10 @@ an event loop:
 
 .. function:: get_event_loop()
 
-   Get the current event loop.  If there is no current event loop set
-   in the current OS thread and :func:`set_event_loop` has not yet
+   Get the current event loop.
+
+   If there is no current event loop set in the current OS thread,
+   the OS thread is main, and :func:`set_event_loop` has not yet
    been called, asyncio will create a new event loop and set it as the
    current one.
 

Misc/NEWS.d/next/Documentation/2020-01-18-15-37-56.bpo-39381.wTWe8d.rst

@@ -0,0 +1,2 @@
+Mention in docs that :func:`asyncio.get_event_loop` implicitly creates new
+event loop only if called from the main thread.